Implemented & tested new markup API. See #95 1) Sequences, Alignment and Blocks now have two new methods: - add_markup(name, markup_class=optional, **kwargs=optional) - remove_markup(name) name refers to the same name as in aln.markups[name] or sequence[i].name It is now explicitly denied to create markups any other way. 2) Markups now have `remove()` method that means 'release all memory that would not be released otherwised, if we just remove markup from the dictionary'. For sequences markups it removes markup attribute from each monomer. 3) Added necessary del sequence_markup[monomer] method. 4) Many base classes have attribute `kind`; for Alignments and Blocks it is 'alignment', for Sequences it is 'sequence' for AlignmentMarkups it is 'alignment_markup' for SequenceMarkups it is 'sequence_markup'. This attribute is crucial for new alignment construction API. 5) Common stuff for MarkupContainers (Alignments and Sequences) is in MarkupContainerMixin.